LED lighting has become a preferred technology in lighting applications, from home, commercial, and industrial applications, as well as outdoor applications such as street lighting, and stadium and arena lighting, replacing older incandescent lighting systems. LED lights are able to provide lighting at a much higher efficacy than incandescent light bulbs using much less input electric power to output a given level of lumens. Efficacy is a ratio of lumens output by the light bulb to power drawn from a power source, such as a commercial electric utility source, and expressed as lumens per Watt (lm/W). For LED light bulbs, losses in the power conversion circuitry that convert the input power (e.g. commercial 120 VAC) to the direct current (DC) used by the LEDs of the LED bulb are included in the power consumed by the light bulb when determining efficacy according to some industry standards such as those set by the “DesignLights Consortium” to qualify retrofit LED lighting products for efficiency rebates. Many commercial electric energy utility suppliers offer rebate programs in conjunction with governmental agencies to encourage replacement of older lighting sources with new high efficacy lighting sources. To qualify for these rebates, light bulb manufacturers must submit their product for testing to determine whether the products meet the required efficacy standard. Of course, if a manufacturer's product qualify for rebates, they have a substantial market advantage over those that do not.
One of the issues that presents a challenge for manufacturers in meeting rebate standards is the wide range in tolerance of the forward operating voltage of the LEDs. The forward operating voltage is the point at which the light output of the LED is at a specified level. Because of the non-linear nature of LEDs, increasing the current through an LED beyond that needed to achieve the forward operating voltage does not produce a corresponding output in light, and results in wasted power. Manufacturers have two options to deal with the wide tolerance in forward operating voltage.
First, in a series of LEDs, a voltage is applied to the series sufficient to ensure that all of the LEDs will be operating at their forward operating voltage, and because the forward operating voltage of LEDs changes over temperature, the voltage applied must be sufficient to ensure the LEDs all operate at their forward operating voltage over their entire specified/intended operating temperature range. This approach results in significant losses of efficacy because most of the LEDs will have a forward operating voltage that is below the highest end of the forward operating voltage range, meaning those LEDs are simply dissipating excess power as a tradeoff to ensure the few LEDs with high forward operating voltages are turned on (producing light). Using this approach it is not uncommon for LED light bulb manufacturers to use a power source that provides a voltage across a series of LEDs on the order of about 3.42 volts per LED.
A second approach is to sort LEDs by forward operating voltage and select only those LEDs having a forward operating voltage in a much narrower range. This approach allows a more precise voltage level be provided to a series of LEDs to ensure they all reach their forward operating voltage across their operating temperature range, which will be much more uniform among the LEDs than when not selecting/sorting them. However, this approach is costly as each individual LED has to be tested and sorted.
Accordingly, there is a need for an inventive technology that avoids the problems associated with prior solutions and allows for the design and manufacture of LED light bulbs and lighting systems that have a high efficacy without incurring an excessive cost.
Those skilled in the field of the present disclosure will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. The details of well-known elements, structure, or processes that would be necessary to practice the embodiments, and that would be well known to those of skill in the art, are not necessarily shown and should be assumed to be present unless otherwise indicated.